Your bigger problem is going to be the deer.

Pine needles used constantly for years as a mulch (as in under a pine tree) will only very slightly acidify soil, but as the needles break down, I believe they end up pretty much neutral - kind of like all composted plant material tends to be.

I grow a lot of things under some slightly limbed up spruce in one of my gardens - hostas which deer will eat but tend to recover, "Dart's Gold" ninebark which is such a vigorous grower it can handle and outgrow nibbling. Ostrich ferns, actaea pachypoda alba, wild columbine, astilbe, bleeding hearts never get touched. On the outer edges with the most sun, I use an old variety of nepeta which has always been reliably safe along with my divided Stella d'Oro daylilies and some sedums - neither of which has been browsed so far this year. In the spring I've got an understory of scilla, muscari, puschkinia, and forget-me-nots - none of which any creature seems to bother.

If you search the web for deer resistant plants, one of the first entries is the New Jersey Agr. Ext. service (NJAES). That has a big chart of plants, trees and shrubs and rates them as to rarely damaged, seldom severely damaged...

Nothing is foolproof, however, given the variables of avaiability of natural food, what deer develop a taste for in any given area etc.

If it's even moderate dry shade, the Astilbe will be disappointing. Ferns are a definite yes. Other plants to consider: Cerastostigma willmottianum, Pulmonaria, Lamium, Geranium maculatum, Tradescantia virginiana, Daffodils, Carex oshimensis 'Evergold', Columbine, Epimedium, Dicentra, and Japanese beautyberry (much more refined than the American species).